Biaxially oriented hollow thermoplastic bodies and improved method for sterilization

ABSTRACT

A process of molding thermoplastic preforms into bottles and similar containers wherein pressurized liquids, such as water, are used in the stretching and shaping process instead of pneumatic gases, such as heated air. The result is greater control over the crystallization of the thermoplastic material and economies of scale. Addition of peroxides, or similar materials are used for sterilization can be added to the liquid thereby conditioning the container for immediate filling with sterile product and eliminating the need for an additional sterilization step following completion of the molding step. As an added step, dry, sterile air can be used to vent and dry the container just prior to introduction of product. Also use of liquid, rather than heated air, provides a washing or cleansing of actealdehydes or ethanol, which may be present in the extruded preform, from the finished container.

BACKGROUND OF THE INVENTION

[0001] Plastic forming of hollow bodies has heretofore encompassed awide variety of processes and techniques including but not limited toblow molding, stretch-blow molding, thermoforming and the like. Forexample, hollow bodies having threaded necks are produced frominjection-molded preforms which are heated and then expanded by highpressure air through a process called blow molding. At times the preformis allowed to cool down prior to arrival at the blow molding stationfrom the injection molding station and then it must be thermallyconditioned within a relatively narrow tolerance to a temperature thatis suitable for a subsequent stretching process. In order to obtainuniform and good orientation, the temperature variation across the wallof the body should be within 10 degrees Celsius and not much above 10degrees Celsius from the glass transition temperature of the plastic tobe stretch-blown during the forming process. The closer the temperatureis to the glass transition temperature, the more orientation isobtained. However, much higher pressure is required to stretch and blowthe container at the lower temperature; therefor, a compromise has to bemade. The oriented hollow bodies thus obtained exhibit a mechanicalstrength which is markedly improved to the point that the bodies cangenerally be used for packaging carbonated beverages under pressure. Theair pressure used in this known process for stretch-blow molding canexceed 1,000 pounds per square inch (psi) if a particular bottom designis required for such containers. Other features, such as molded-inimpressions, horizontal rings to resist buckling of the container, andvertical lines to improve stackability all require even higher airpressure during the forming process.

[0002] Heat-setting of polyethylene terephthalate (PET) bottles, orother shapes of containers, by stretch-blowing the preforms in a hightemperature mold is presently used to crystallize the PET. This processis complicated and requires long cycle times. The container is firstheated and then cooled by the mold. Therefore, the larger the change intemperature, in order to speed the crystallization, the more energy isconsumed during the process, and compromises must be made between theenergy use and the speed of the process.

[0003] Also, it is known in the prior art that the containers, such asplastic bottles used for beverages, and containers for food products andthe like for human or animal consumption, need to be sterilized to avoidcontamination of the food, beverages and the like. Heretofore, thissterilization process required an additional process step followingcompletion of the container forming and following the step of exhaustingthe pressurized air used during the blow molding process.

[0004] Another shortcoming of the prior art is that during the extrusionof the plastic material and the shaping of the preform, formation ofactealdehydes, or ethanol, occurs. These products are toxic in natureand can remain in the container body following the use of pressurizedair during the blow molding process.

SUMMARY OF THE INVENTION

[0005] The present invention utilizes liquids instead of air or othergases for the forming process. Liquids are not compressible in a moldingenvironment; therefore; liquids require less energy to transfer a load,and at the same time liquids are excellent heat transfer media. Becausethe invention is particularly useful for forming containers to be usedin the food and beverage field, the present invention is particularlyadaptable to the use of water as the hydraulic media. Water is anexcellent hydraulic and heat transfer media, economic, readilyavailable, non-toxic, non-hazardous, and subject to easy handling.

[0006] Another advantage of using liquids is the avoidance ofintroducing oxygen into the body of plastic material such as occurs inthe prior art in which air is used as the pressure medium. Oxygen has adeleterious effect on many products by reducing shelf-life and causingdeterioration of quality. Use of water or other liquids as the pressuremedium avoids or minimizes this problem.

[0007] The present invention is useful for forming hollow bodies in theform of containers, or bottles, for liquids such as beverages and thelike, and also useful for containing solid products such as granularmaterials, powders, and materials such as pastes, waxes, depending uponthe size and shape of the container and the container opening used tofill and empty product to and from the container.

[0008] The present invention relies upon the use of hydraulic media,such as water, rather than upon pneumatic media such as air that hasbeen used in the past. The work required to create 1,000 psi airpressure is one order of magnitude higher than the work required tocreate 1,000 psi hydraulic pressure. The equipment to perform the workis proportionate in cost. Both the capital equipment cost and the energycost constitute a significant portion of the total cost of producing thebottle or other form of container. Because the cost to create highpressure by hydraulic means is not expensive, then it is feasible tostretch thermoplastic preforms at lower temperatures that are closer tothe glass-transition of the material. This improved process results inbetter molecular orientation and improved properties leading to theproduction of lighter weight containers using less material.

[0009] In accordance with the present invention, sterilization of thecontainers does not require a separate process step, as is required bythe prior art. The hot liquid used for forming the container aidssomewhat in sterilization while the container is being formed.Sterilization can be assured by introducing sterilization products intothe hot water, or other liquid so that the container is sterile uponcompletion of the stretch molding step ready to receive product uponbeing emptied of the liquid used during the forming step. Dry sterileair may be used to vent and dry the container, if needed, following theforming step.

[0010] A further feature of the present invention is that by the use ofliquid during the expansion of the preform into the desired containershape, the liquid has a washing or cleansing effect to remove some orall of the actealdehydes or ethanol that was formed during the processof extruding the preform.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The invention is particularly adaptable to the production ofpolyethylene terephthalate (PET) containers, such as bottles and thelike. The novel process can be adapted to the present manufacturingprocess with minor but important changes. Instead of using hot,pressurized air to blow the preform, tempered water, at approximately 85degrees Celsius, is pumped into the preform and then pressurized. Whenthe water pressure reaches the desired value and the preform hasexpanded or stretched to form the desired container shape, the hot wateris vented and cooler water is pumped into the bottle, or other form ofcontainer, to replace the hot water that can then be stored and used forthe next cycle. It is contemplated that the water, or other liquid usedduring the forming or expansion process can be supplemented by productsutilized for sterilization. It is presently preferred that the formingliquid be laced with, for example, peroxide(s) or acetic acid, or otherproducts that are approved by the Food and Drug Administration (FDA).Following the forming step, the bottle or other container then may, butnot necessarily, be inverted and sterile, dry air is blown into thebottle to efficiently empty any remaining water, thus leaving thecontainer in a sterile condition and ready for immediate filling withproduct without requiring the additional sterilization processheretofore needed by the prior art of blow molding with pressurized air.Thus, the containers formed with the present invention can beimmediately filled with sterile products, or food or beverages for humanconsumption.

[0012] In those instances in which heat setting is required to make thecontainers suitable for heat processing of product contained therein,the mold should have the capability to be heated close to the meltingpoint of the thermoplastic material. This could be accomplished, forexample, by rf-heating or induction heating. It would be unnecessary tohave a cooling arrangement because the required cooling would beachieved by the internal liquid which, as pointed out above, may be atapproximately 85 degrees Celsius and, therefor, below the melting pointof the plastic used to form the container.

[0013] The foregoing is a description of the presently preferredembodiment of the invention but it is to be understood that variationsmay be made within the scope of the invention as defined in the appendedclaimed subject matter.

1. A process of forming a container from thermoplastic materialincluding the steps of supplying a preform for said container, placingsaid preform within a cavity of a mold, and expanding said preform forforming said container, characterized by the step of introducing liquidunder pressure into said preform located within said mold for stretchingsaid preform to assume the shape of the surrounding mold cavity.
 2. Aprocess as defined in claim 1 including a step of heating said liquid toapproximately 85 degrees C.
 3. A process as defined in claim 1 includinga step of heating said mold to approximately the melting point of thethermoplastic material.
 4. A process as defined in claim 3 wherein heatis transferred from the mold, through the thermoplastic material intocooler liquid introduced within the container.
 5. A process as definedin claim 4 wherein the crystallization of the thermoplastic material canbe increased or decreased under controlled conditions.
 6. A process asdefined in claim 5 including a step of introducing a sterilizationproduct into said liquid.
 7. A process as defined in claim 1 including astep of introducing a sterilization product into said liquid.
 8. Aprocess as defined in claim 7 wherein said sterilization product is aperoxide.
 9. A process as defined in claim 4 including a step ofpreheating the liquid to approximately 85 degrees C., prior tointroducing said liquid into said preform.
 10. A process as defined inclaim 1 including a step of introducing dry, sterilized air into saidcontainer for venting and drying said container.
 11. A process asdefined in claim 2 including a step of venting said liquid from saidcontainer and recycling said liquid for subsequent use.
 12. A process asdefined in claim 11 including a step of introducing dry, sterile airinto said container prior to a step of introducing product into saidcontainer.
 13. A process as defined in claim 1 including a step ofheating said thermoplastic material to a temperature close to itsmelting point, and introducing treated liquid into said preform.
 14. Aprocess as defined in claim 13 including a step of treating said liquidby including a peroxide.
 15. A process as defined in claim 15 includinga step of mixing said peroxide with water for forming said liquid.
 16. Aprocess as defined in claim 1 wherein said liquid cleanses said preformof actealdehydes and ethanol.
 17. A process as defined in claim 7including the step of filling said container with product uponcompletion of forming said container.